1. Field of the Invention
The present invention relates to an image forming apparatus; such as, a copying machine or a laser printer, or more particularly, to an image forming apparatus using image formation control inference.
2. Related Background Art
FIG. 1 shows an example of a schematic configuration of a color copying apparatus of a conventional color image forming apparatus. In FIG. 1, numeral 1 denotes an original. 2 is an original glass base on which the original is placed. 3 denotes a light. 4 represents an image forming element array. 5 is an infrared cutoff filter. 6 denotes a contact type CCD color sensor (hereafter, CCD). 7 is an optical unit.
First, a copying process will be described. When a Copy key (not shown) is pressed, a light 3 illuminates an original 1. Light reflected from the original passes through an image forming element array 4 and an infrared cutoff filter 5. Then, the original image is formed on a CCD 6. An optical unit 7 moves to scan the original in an arrow-direction. As shown in FIG. 2, the CCD 6 has red (R), green (G), and blue (B) filters set in an array for each pixel.
While scanning the original, an electric signal from the CCD 6 is processed by a signal processing circuit shown in FIG. 3. In FIG. 3, 6R, 6G, and 6B denote signals sent from R (red), G (green), and B (blue) elements on the CCD 6. Next, the R, G, and B original image signals go to a circuit 14 for logarithmic or A/D conversion, then are converted into digital signals Y', M', and C' in association with formation image colors; that is, yellow (Y), magenta (M), and cyan (C). The Y', M', and C' signals enter a color conversion circuit 15 for masking or underlying color removal (hereafter, UCR). The color conversion circuit 15 performs an operation represented as the expression below. ##EQU1## where, (Y', M', C').sub.min is the smallest among Y', M', C and C' signals, and a.sub.11 to a.sub.44 and b.sub.4 are color conversion coefficients.
Thus, the color conversion circuit 15 provides Y", M", C", and B.sub.K " signals. These signals enter a color laser printer 16 to drive a laser driver (not shown).
FIG. 4 shows an example of a configuration of a color laser printer 16 that has been known in the past. In FIG. 4, a laser beam converted by a laser driver (not shown) is emitted to scan photosensitive drum 11 via a scanning polygon mirror 8 and a stationary deflection mirror 9. Then, a latent image is produced on the photosensitive drum 11 that is rotating in an arrow direction. The latent image is developed using color toner with rotation of a rotary developing unit 10.
On the other hand, transfer paper 13 is wound on a transfer drum 12. The transfer drum 12 rotates once for each of colors Y, M, C, and B.sub.K in that order, thus rotating four times in total. When transfer is complete, the transfer paper 13 departs from the transfer drum 12. Then, fusing rollers 22 fuse the toner of the colors. Thus, a print 20 is created.
In the aforesaid example of prior art, fixed values are used as the color conversion coefficients (coefficients a.sub.11 to a.sub.44, and b.sub.4 in the expression (1)) for a color conversion (masking and UCR) circuit 15. That is to say, these color conversion coefficients are predetermined to minimize color differences between an original 1 and a print 20.
However, the color processing characteristics of a printer 16 vary depending on a toner feed state, or with an environmental variation or a time-sequential change of a photosensitive member. This makes it difficult to retain uniform color reproducibility. Thus, when the color conversion coefficients for a color conversion circuit are fixed, the color differences between an original 1 and a print 20 become noticeable.
To cope with the above problems, this applicant has proposed a solution in Japanese Patent Application No. 63-208369. In the solution, the result of output of a specific gradation pattern is read out to set the color processing characteristics of an image printer. However, it is time-consuming that a specific pattern must be output, then the result must be read.
In the aforesaid image forming apparatus, a means for correcting the gamma characteristic of a printer is incorporated to divide gradation in steps of densities. However, such characteristics as the sensitivity of a photosensitive drum, development capability, and transfer efficiency vary with an environmental variation or a change over time. Preprogrammed gamma correction cannot provide satisfactory gradation in the initial stage. In U.S. Pat. No. 4,888,636 or Japanese Patent Application No. 63-208368, this applicant has proposed an image forming technology using an operation in which an image is produced according to a specific pattern on a photosensitive drum or recording medium, the output of a pattern reading means is compared with an initial gradation pattern, then the gradation correction characteristic of a gradation correcting means is determined.
However, in the foregoing example of prior art, it is time-consuming to perform gradation correction after an image is formed according to a specific pattern. Furthermore, toner is wasted. In particular, when gradation correction must be done frequently, a time loss caused by gradation correction becomes serious. Moreover, a procedure of detecting the variations of the elements or performances of a photosensitive drum, a developing unit, and a transfer drum, then feeding back the variations to a gradation correcting means requires a complex control program.